Rule – 4, Unrealised rent

⁸⁶4. For the purposes of the Explanation below sub-section (1) of section 23, the amount of rent which the owner cannot realise shall be equal to the amount of rent payable but not paid by a tenant of the assessee and so proved to be lost and irrecoverable where,—

(a) the tenancy is bona fide;
(b) the defaulting tenant has vacated, or steps have been taken to compel him to vacate the property;
(c) the defaulting tenant is not in occupation of any other property of the assessee;
(d) the assessee has taken all reasonable steps to institute legal proceedings for the recovery of the unpaid rent or satisfies the Assessing Officer that legal proceedings would be useless.

Reference: www.incometaxindia.gov.in